(a) Field of the Invention
The present invention relates to a shadow mask for a cathode ray tube. More particularly, the present invention relates to a shadow mask for a cathode ray tube, which is large and has a panel with a flat front surface, and to a cathode ray tube having such a shadow mask.
(b) Description of the Related Art
A cathode ray tube (CRT) is used as an image device for a television, computer monitor, etc. A shadow mask used in a CRT performs a color selection function, in which selection of electron beams emitted from an electron gun is performed so that the electron beams land correctly on a phosphor surface. The shadow mask is structured corresponding to a basic size and shape of a front glass panel of the CRT. Shadow masks also typically have a radius of curvature of approximately R=2,000 mm.
In response to consumer demand, CRTs are becoming larger in size and are formed having a flatter front panel surface. The shadow mask used in such CRTs must undergo corresponding changes in dimension and shape. That is, in large-size CRTs that use a panel with a flat outer surface and a curved inner surface, a shadow mask is used that corresponds to the size of the panel and that is curved similarly to the same.
However, the shadow mask becomes structurally weak if it is made to a large size and having a large radius of curvature, leading to various problems. For example, if a radius of curvature of the shadow mask is made to 1.6R, the shadow mask is unable to easily maintain its shape in the event the shadow mask receives a certain level of shock. Such deformation of the shadow mask greatly reduces the quality of the CRT.
Further, when the shadow mask is large and has a flattened curvature, the shadow mask becomes susceptible to howling. That is, in the case where the CRT is used in a large color television, the shadow mask of the CRT vibrates from the sound generated by the speakers of the television. With the increased size of the shadow mask, it becomes structurally weak as described above such that the shadow mask is more prone to howling.
In order to make the shadow mask resistant to physical shock, various approaches are being used, such as the curved surface of the shadow mask being formed in a long axis direction and a short axis direction satisfying specific equations, and the shadow mask being formed with certain limitations with respect to the radius of curvature in the long and short axis directions. Examples of CRTs utilizing such configurations include the CRT disclosed in U.S. Pat. No. 5,606,217 and the CRT disclosed in Japanese Laid-Open Patent No. 2001-319600.
Further, a thickness of the panel to which the shadow mask is mounted is adjusted to enhance shock characteristics of the shadow mask. In particular, peripheral portions of the panel are made greater in thickness than a center portion of the same (approximately two times thicker or more), and the shadow mask is formed having a corresponding curvature, thereby minimizing damage that may be caused by external shock. However, by this formation of the panel in which the peripheral portions are made thicker than the center portion thereof, the overall weight of the CRT increases. This makes manufacturing of the CRT more difficult and may inconvenience users when moving the display system that includes the CRT.
In addition, if an optimum thickness ratio between the center and periphery of the panel while considering the shock characteristics of the shadow mask cannot be obtained. That is, if the thickness at peripheries is too large compared to the thickness of the center portion of the panel, it becomes necessary to form a coating film, which adjusts transmissivity, on a front surface of the panel in order to prevent deterioration of contrast characteristics of the CRT caused by the transmissivity of the glass forming the panel. This extra step of forming the coating film complicates the overall manufacturing process, ultimately increasing CRT unit costs.